Individuals infected with COVID-19 are most likely to transmit the virus to close contacts two days before or three days after symptoms manifest, and the risk of transmission is greatest when patients have mild or moderate disease severity, according to a new study from the University of Georgia.

The findings, published in JAMA Internal Medicine, supports the notion that when a person with COVID-19 is sicker, they are more infectious than asymptomatic patients.

The study adds more evidence for interventions such as contact tracing, masking, and vaccinations, according to main author Yang Ge, a doctorate student in the College of Public Health at the University of Georgia.

“We found asymptomatic cases had lower transmissibility compared to symptomatic cases and were less likely to infect their contacts. In addition, we found that contacts that developed COVID-19 infections were more likely to be asymptomatic if they were exposed to an asymptomatic case,” said Ge.

“This suggests interventions like vaccines and masking should continue to be encouraged.”

Vaccines not only protect the vaccinated individual, but also function to minimize the amount of virus that close contacts may be exposed to, and masking helps prevent the spread of aerosolized particles containing the virus.

The research team drew its conclusions from a large cohort analysis of 730 persons diagnosed with COVID-19 in Zhejiang Province, China, between January 8, 2020, and July 30, 2020.

Using comprehensive health records and contact tracing, the team was able to apply cutting-edge analytical techniques to establish how exposure timing and disease severity affected the chance of transmission.

The group included 8,852 close contacts, which were characterised as family members, coworkers, and individuals exposed in a health care setting or shared transit.

This is the largest contact tracing study to date, according to corresponding author Ye Shen, an associate professor of epidemiology and biostatistics in the school of public health.

Though Shen notes that these findings must be replicated in vaccinated groups, the study highlights a high-risk transmission window that can assist local municipal and public health officials in focusing contact tracing efforts.

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